Condenser Pressure Control System and Method

ABSTRACT

A method of controlling a condenser fan of a heating, ventilating, and air-conditioning (HVAC) system based on a comparison of a refrigerant flow rate to maintain a valve position of an expansion valve. In the method, a controller modulates the condenser fan to maintain the valve position of the expansion valve at a valve position setpoint when the refrigerant flow rate is higher than the critical flow rate. The method also comprises controlling the speed of the condenser fan of the HVAC system to maintain a condensing measurement at a plurality of condensing measurement setpoints when the refrigerant flow rate is higher than the critical flow rate. The method also comprises controlling the speed of the condenser fan to maintain a condensing measurement at a plurality of condensing measurement setpoints when the condensing temperature measurement is higher than an ambient air temperature value plus at or around 5° F.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not Applicable

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX

Not Applicable

TECHNICAL FIELD

The disclosed embodiments generally relate to a system and method for controlling condenser fans, and more particularly for controlling condenser fans in HVAC (heating, ventilation, and air conditioning) units as well as commercial and industrial refrigeration system applications.

DESCRIPTION OF THE RELATED ART

Condensers are devices that condense a substance from a gaseous state to a liquid state. Air-cooled condensers enable this phase change using ambient air, while water cooled condensers use water as the cooling means. They are a common component of air conditioning and refrigeration systems. A condenser control system may include at least one VFD (variable frequency drive) for modulating the speed of the condenser fan (in air-cooled condensers) or a control valve for controlling the water flow rate (in water-cooled condensers). Condenser control systems generally comprise at least one temperature or pressure sensor that measures the temperature or pressure, respectively, of a refrigerant at the liquid outlet pipe of the condenser. In typical control systems, temperature sensors are located in the middle, outlet, or inlet pipes of the condenser, while pressure sensors are typically located just in the outlet pipe of the condenser. When the ambient air temperature (in the case of air-cooled condensers) or water temperature (in the case of water-cooled condensers) is high, the pressure in the condenser is determined by the design of the condenser. The condenser pressure can also be determined by taking the temperature difference between the refrigerant and the outside air while the fan (in the case of air-cooled condensers) is operating at full speed or the control valve (in the case of water cooled condensers) is in a fully open position.

When the ambient air temperature (in the case of air-cooled condensers) or water temperature (in the case of water-cooled condensers) is low, the pressure in the condenser must be high enough so that the rated refrigerant can flow through the expansion valve. This is the defining principle for determining the minimum condensing pressure, also called the head pressure. The minimum head pressure can be found by summing together the total pressure in the evaporator with the pressure drop across the expansion valve (under the design conditions) and the pressure drop in the pipe located between the condenser & evaporator. There are a variety of ways in which the controller of the condenser can maintain the minimum head pressure. It may turn the fan on and off, modulate the fan speed, and/or (in applications having water-cooled condensers) open/close a two position valve.

Since the head pressure setpoint significantly influences the compressor power, scholars in the prior art have developed a method called the floating head pressure control method to save energy. The optimal head pressure set point in the floating pressure control method is determined based on the ambient air temperature and rated refrigerant flow rate. The optimal head pressure is set at or higher than the minimum head pressure. As an example, when a refrigerant such as monochlorodifluoromethane (R-22) is used in a system employing the floating head pressure method, the suction pressure is typically set at 70 Psig. At the design flow rate, the design pressure drop for the expansion valve is supposed to be about 100 Psi, while the design liquid pressure drop is about 5 Psi. The minimum head pressure set point should thus be set to 175 Psig. Under real system conditions, the minimum head pressure set point is often set from 195 Psig to as high as 250 Psig. The corresponding condensing temperature becomes about 100° F. and 118° F., respectively. However, the majority of the time the refrigerant flow rate is less than the rated flow rate under partial load conditions. As a result of the high condensing pressure, when the flow rate of the refrigerant is less than the design rate, the expansion valve is configured to partially close. This in turn results in the excessive consumption of compressor power.

There are a variety of methods proposed in the prior art to control the operation of the condenser fan in air-cooled condensers or the cooling water valve in water-cooled condensers. It is common to use the sensed temperature and pressure values to control the fan or cooling water valve under low ambient temperatures. In a prior art temperature based fan control method, for example, a temperature sensor is installed in the condenser fan control system. Using the readings from this sensor, the controller of the condenser fan control system can be programmed to compare the measured condensing temperature with the set point temperature. Based on the comparison, in some applications the controller is programmed to command the fan on or off or the control valve open or closed to maintain the condensing temperature to within specific high/low temperature limits (for the R-22 refrigerant, for example, this is 100° F. to 118° F.). In other applications, the controller is programmed to modulate the fan speed (for air-cooled condensers) or control the valve position (for water-cooled condensers) to maintain the condensing temperature at a single, pre-set setpoint value (For R-22, this is 100° F.).

The controller activates or deactivates the fan and/or opens or closes the control valve to maintain the pressure of the refrigerant to within the specified high and low limits of the setpoint (195 Psig to 250 Psig for R-22). FIG. 1 is an illustration of a prior art condenser fan control system. In the figure (FIG. 1, System 100), condenser 106, pressure sensor 112, expansion valve 114, evaporator 116, compressors 118 and 119, condenser fan 120, controller 122, thermal bulb 124, and variable frequency drive (VFD) 140 are connected via conduit in a typical refrigeration system. Any type of compressor may be employed (including constant speed and multiple speed compressors). Condenser fan 120 (in condenser 106) of the refrigeration system is connected in communication with controller 122. Controller 122 is also configured in connection with pressure sensor 112. In the prior art method, controller 122 collects pressure values from pressure sensor 112 and modulates the speed of condenser fan 120 based on those values. Variable frequency drive 140 is modulated to maintain the condensing pressure at a single setpoint.

Thermal bulb 124 is also configured within system 100 at the tailpipe of evaporator 116 so that it is in connection with and can be used to control the opening and closing of expansion valve 114. This temperature sensing bulb controls the flow of refrigerant through the refrigeration system. The bulb is filled with a gas that is the same as the gas in the refrigeration system. As the temperature in the bulb increases, the gas in the bulb expands and puts pressure on expansion valve 114. The pressure causes expansion valve 114 to open, thereby increasing the flow of refrigerant through the system. When the temperature in the suction line of the refrigeration system decreases, the pressure of the gas in thermal bulb 124 also decreases causing expansion valve 114 to close. Closure of valve 114 results in a restriction in the flow of refrigerant.

While this prior art system is functional, there are certainly problems with controlling the condenser fan to maintain the condensing temperature or pressure at a single setpoint. For one, the amount of refrigerant that flows through the expansion valve and liquid pipe line may be much smaller than the design value when refrigeration systems use or are equipped with the following:

-   -   1. They employ a VFD (variable frequency drive) to modulate the         speed of the compressor     -   2. They have a small number of operating compressors in a system         equipped with multiple compressors     -   3. They have multiple stage reciprocating compressors     -   4. They employ a slide valve to modulate the capacity     -   5. They use a high temperature liquid or hot gas for reheat         purposes     -   6. They have a hot gas by-pass (whereby hot gas is bypassed         through the high pressure valve and returned to the inlet of the         compressor).

Under all of the previously stated conditions, the flow rate of the refrigerant that flows through the expansion valve may be as low as 30% of the design flow rate. Table 1 below lists the pressure loss across the expansion valve and pipelines as well as the evaporator pressure of the R-22 refrigerant for different flow rates. Based on the given information, it is possible to determine the optimal head pressure at each flow rate. When the flow rate is reduced from 100% to 50%, for example, the minimum head pressure set point is reduced from 178 Psig to 96 Psig. At the same time, the liquid temperature set point is reduced from 87° F. to 55° F. The compressor head lift can be reduced by as much as 75%. In real system applications in which the R-22 refrigerant is employed, the minimum floating head pressure can be as high as 190 Psig. The fixed head pressure set point is often set as high as 250 Psig. Therefore, optimizing the head pressure can significantly reduce the power of the compressor. It is important to point out that the condensing temperature (also called the optimal head pressure) varies according to the refrigerant flow rate and is thus not a fixed value. An additional problem is that under the design head pressure the expansion valve can malfunction when the refrigerant flow rate is low.

TABLE 1 Variance in the pressure values of a refrigeration system using the R-22 refrigerant under differing refrigerant flow rates. Flow 50% 60% 70% 80% 90% 100% 120% 140% Expansion valve loss 25 36 49 64 81 100 144 196 Pipe loss 2.5 3.6 4.9 6.4 8.1 10 14.4 19.6 Evaporator pressure 83 83 83 83 83 83 83 83 Optimal Psia 111 123 137 153 172 193 241 299 Head Psig 96 108 122 139 157 178 227 284 pressure Bar 6.7 7.6 8.6 9.8 11.1 12.6 16.0 20.0 Liquid Temp 55.0 60.0 65.0 72.0 79.0 87.0 105.0 125.0 Lift reduction 75% 64% 51% 36% 19% 0% −44% −96%

In addition, while thermal bulb 124 is able to control the opening and closing of expansion valve 114, by itself it is not able to maintain the position of expansion valve 114 at an optimal fully open position for long periods of time. Therefore, in order to overcome the problems posed in the prior art, a new system is proposed herein. The following describe key advantages of embodiments of this new system: In the embodiments, the expansion valve is connected in direct communication with a controller. This controller is configured with a control algorithm that ensures that the expansion valve is constantly maintained in an optimal open position and can thus optimally circulate refrigerant throughout the refrigerant system. Prior art systems, such as the one shown in FIG. 1, tend to rely solely on the thermal bulb to control the position of the expansion valve and are thus not able to optimally regulate the flow of refrigerant. In addition, another key advantage of the disclosure is that in at least some of the disclosed embodiments, the proposed method saves more energy than the prior art methods since it is able to modulate the condensing temperature or pressure at multiple setpoints rather than at a single setpoint. Other related advantages of the proposed embodiments disclosed herein are summarized in the following:

It is therefore an advantage of an embodiment to reduce the excessive consumption of compressor power under partial load conditions and to extend the lifetime of the compressor.

It is another advantage of an embodiment to reduce the power of the condenser fan and condenser pressure under partial load conditions to extend the lifetime of the condenser.

It is yet another advantage of an embodiment to improve the energy efficiency in terms of the coefficient of performance (COP).

It is a further advantage of an embodiment to reduce excessive pressure on the expansion valve. The reduction results in reduced O&M costs, and ensures that the HVAC system is operating smoothly.

SUMMARY OF THE INVENTION

The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to an embodiment of the present invention and is not intended to be a full description. A full appreciation of the various aspects of the embodiments can be gained by taking the entire specification, claims, drawings, and abstract as a whole. The embodiments described in this disclosure offer some key advantages of the invention over the prior art as summarized below:

In one embodiment, a method of controlling a condenser fan to maintain a valve position of an expansion valve of a heating, ventilating, and air conditioning (HVAC) system at a valve position setpoint is proposed. The method involves providing a controller in communication with the expansion valve of the HVAC system. The controller is operable to receive a signal indicating the valve position. It also involves providing a refrigerant flow rate sensing device in communication with the HVAC system and controller operable to measure a refrigerant flow rate of the HVAC system. The method further entails configuring the controller with a critical refrigerant flow rate value and the valve position setpoint. The controller compares the refrigerant flow rate with the critical refrigerant flow rate value and modulates the condenser fan to maintain the valve position of the expansion valve at the valve position setpoint when the refrigerant flow rate is higher than the critical refrigerant flow rate.

In another embodiment, a method of controlling a condenser fan of a heating, ventilating, and air-conditioning (HVAC) system to maintain a liquid condensing measurement of a refrigerant at a plurality of condensing measurement setpoints is proposed. The HVAC system comprises at least one evaporator, expansion valve, condenser, and compressor configured in a refrigerant circuit. The method involves providing a condensing measurement device in connection with the condenser and controller and is configured to provide a condensing measurement. The method further entails providing a refrigerant flow sensing device in communication with the controller and HVAC system that is operable to measure a refrigerant flow rate value. The method further comprises determining, by the controller, a system load ratio (ω) for the HVAC system. It involves programming the controller with a plurality of variables for the HVAC system comprising a design flow rate value of said refrigerant, the system load ratio value (ω), a critical flow rate value of the refrigerant, a subcooling temperature of the refrigerant, a saturated pressure measurement corresponding to the evaporative temperature (P_(evaporator)) of the evaporator; and a sum of a pressure loss value from said expansion valve and a pressure loss value in a liquid line of the refrigerant circuit at said design flow rate value (ΔP). The method further involves determining, by said controller, a condensing pressure setpoint (P_(set)) of the condenser based on said plurality of variables, wherein P_(set)=P_(evaporator)+ω²ΔP, and modulating, by the controller, the speed of the condenser fan to maintain the condensing measurement value at the plurality of condensing measurement setpoints when the flow rate value of the refrigerant is higher than the critical flow rate value.

Some embodiments of the method comprise providing a pressure sensor in communication with the controller and operable to measure and send to the controller a pressure measurement from the liquid line of the refrigerant circuit. The controller modulates the speed of the condenser fan to maintain the pressure measurement at a plurality of condensing pressure setpoints when the refrigerant flow rate value is higher than the critical flow rate value. The condensing measurement setpoint is the condensing pressure setpoint (P_(set)). Other embodiments of the method comprise providing a temperature sensor in communication with and operable to measure and send a temperature measurement to the controller. The controller modulates the speed of the condenser fan to maintain the temperature measurement at a plurality of condensing temperature setpoints when the refrigerant flow rate value is higher than the critical flow rate value. The plurality of condensing temperature setpoints is defined as the saturated temperature value of the refrigerant at the condensing pressure setpoint(P_(set)).

In some embodiments of the method the sensing device is a flow meter operable to measure a refrigerant flow rate, and the step in which the controller determines the system load ratio (ω) for the HVAC system further comprises dividing the refrigerant flow rate value over the design refrigerant flow rate value.

In other embodiments of the method, the sensing device is a compressor status device operable to measure a refrigerant flow rate as well as to collect and transmit to the controller a signal indicating when the compressor of the HVAC system is in an active state of operation. When this method is used, the HVAC system must have at least two compressors connected in communication with the controller and the step in which the controller determines the system load ratio (ω) for the HVAC system further comprises transmitting, by the compressor status device, the compressor status signal for the compressors to the controller. The controller divides a sum of the compressor status signal for the compressors in operation over a sum of the compressors in the HVAC system.

In still yet other embodiments of the method, the sensing device is a compressor speed device operable to measure a refrigerant flow rate as well as to collect and transmit to the controller a signal indicating a speed of the compressor. In the method, the step in which the controller determines the system load ratio (ω) for the HVAC system further comprises providing the controller with a compressor design speed and transmitting, by the compressor status device, the signal indicating the speed of the compressor to the controller. The step further entails dividing, by the controller, the speed of the compressor over the compressor design speed.

In another embodiment, a method of controlling a condenser fan of a condenser of a heating, ventilating, and air-conditioning (HVAC) system to maintain a condensing measurement value at a plurality of condensing measurement set points based on an ambient air temperature measurement is proposed. The HVAC system has at least one evaporator and expansion valve configured in a refrigerant circuit. The method involves providing a controller in communication with the HVAC system and providing an ambient air temperature sensor in communication with and operable to measure and send to the controller an ambient air temperature value (T_(amb)). It also comprises providing a condensing measurement device in communication with the controller and operable to measure and send to the controller a condensing measurement value. The method involves determining, by the controller, a condensing temperature value (T_(cond)). It also entails programming the controller with a ratio of a speed of the condenser fan over a design speed of the condenser fan (ω) a design condenser split temperature value of the HVAC unit (ΔT), an on/off heat transfer coefficient ratio for the condenser fan (α), a suction pressure value (P_(suc)) from the evaporator, a subcooling temperature of the refrigerant, and a sum of a pressure loss value across a liquid line and a suction line of the refrigerant circuit and a pressure loss at the expansion valve of the HVAC unit under a design flow rate (ΔP_(d)). Using those variables, the controller is configured to determining a cooling load ratio (β) for the condenser, wherein:

$\beta = \left\{ \begin{matrix} {\frac{T_{cond} - T_{amb}}{\Delta \; T}\omega^{0.76}} & {\omega > 0.1} \\ {\frac{T_{cond} - T_{amb}}{\Delta \; T}\alpha} & {\omega \leq 0.1} \end{matrix} \right.$

The controller determines a condensing liquid pressure set point (P_(cond.set)) of the refrigerant based on the cooling load ratio (β), wherein:

P _(cond.set) =P _(suc)+β² ΔP _(d)

The method further entails determining, by the controller, a plurality of condensing measurement setpoints based on a condensing measurement value and the condensing liquid pressure set point (P_(cond.set)). It also involves modulating, by the controller, the speed of the condenser fan to maintain the condensing measurement at the plurality of condensing measurement setpoints when the condensing temperature value is higher than the ambient temperature value plus at or around 5° F.

In some embodiments of the method, the step of determining, by the controller, the condensing temperature value (T_(cond)), further comprises providing a temperature sensor in communication with and operable to measure and send to the controller the condensing temperature value (T_(cond)) of the refrigerant. Other embodiments of the method comprise providing a pressure sensor in communication with the condenser and controller and operable to measure and send a condensing pressure measurement value (P_(cond)) to the controller. The method further entails finding a saturated temperature of the refrigerant at the condensing pressure value (P_(cond)).

In some embodiments of the method, a temperature sensor is provided in communication with and operable to measure and send to the controller a liquid temperature condensing measurement value (T_(cond)). The condensing measurement setpoint is determined as a corresponding saturated refrigerant temperature at the condensing pressure setpoint (P_(cond.set)). The controller modulates the speed of the condenser fan to maintain the temperature measurement value (T_(cond)) at the plurality of condensing temperature measurement setpoints when the temperature measurement value (T_(cond)) is higher than the ambient air temperature value plus at or around 5° F. (this temperature value is not limited to 5° F. and is thus adjustable).

In other embodiments of the method, a pressure sensor is provided in communication with and operable to measure and send to the controller a liquid pressure condensing measurement value (P_(cond)). The condensing measurement setpoint is determined as the condensing pressure setpoint (P_(cond.set)). The controller is configured to modulate the speed of the condenser fan to maintain the pressure measurement value (P_(cond)) at condensing measurement setpoint (P_(cond.set)) when the pressure measurement value (P_(cond)) is higher than said saturated pressure under the ambient temperature value plus at or around 5° F. (this temperature value is not limited to 5° F. and is thus adjustable).

Finally, in certain embodiments in which the pressure sensor is implemented, the method further comprises inactivating, by the controller, the condenser fan when the condensing pressure value (P_(cond)) is less than the corresponding saturated pressure value at the ambient air temperature value plus at or around 5° F. In certain other embodiments in which the pressure sensor is implemented, the method further comprises activating, by the controller, the condenser fan when the condensing pressure value (P_(cond)) is higher than said corresponding saturated pressure value at the ambient temperature value plus at or around 5° F. In certain embodiments in which the temperature sensor is implemented, the method further comprises activating, by the controller, the condenser fan when the condensing temperature value (T_(cond)) is higher than the ambient air temperature value plus at or around 5° F. In certain other embodiments in which the temperature sensor is implemented, the method further comprises inactivating, by the controller, the condenser fan when the condensing temperature value (T_(cond)) is less than the ambient air temperature value plus at or around 5° F.

BRIEF DESCRIPTION OF THE DRAWINGS

It will be appreciated that for simplicity and clarity of illustration, elements illustrated in the following figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to other elements for clarity. Advantages, features and characteristics of the present disclosure, as well as methods, operation and functions of related elements of structure, and the combination of parts and economies of manufacture, will become apparent upon consideration of the following description and claims with reference to the accompanying drawings, all of which form a part of the specification, wherein like reference numerals designate corresponding parts in the various figures, and wherein:

FIG. 1 is a schematic diagram of a prior art system.

FIG. 2 schematically illustrates a system embodying the principles of the disclosure in the air-cooled expansion valve based condenser fan system configuration.

FIG. 3 is a schematic diagram illustrating an alternative embodiment of FIG. 2 of the disclosure in the air-cooled expansion valve based condenser fan system configuration.

FIG. 4 is a schematic diagram illustrating an additional alternative embodiment of FIGS. 2 and 3 in the air-cooled expansion valve based condenser fan system configuration.

FIG. 5 is a schematic diagram illustrating a system embodying the principles of the disclosure in the condenser based fan system configuration.

FIG. 6 is a schematic diagram illustrating an alternate embodiment of FIG. 5 of the disclosure in the condenser based fan system configuration.

FIG. 7 is a schematic diagram illustrating a system embodying the principles of the disclosure in the compressor based fan system configuration.

FIG. 8 is a schematic diagram illustrating an alternative embodiment of FIG. 7 of the disclosure in the compressor based fan system configuration.

Before the present methods, systems, and materials are described, it is to be understood that this disclosure is not limited to the particular methodologies, systems, and materials described, as these may vary. It is also to be understood that the terminology used in the description is for the purpose of describing the particular versions or embodiments only, and is not intended to limit the scope.

It must also be noted that as used herein and in the appended claims, the singular forms “a”, “an,” and “the” include plural references unless the context clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art. Although any methods, materials, and devices similar or equivalent to those described herein can be used in the practice or testing of embodiments, the preferred methods, materials, and devices are now described. Nothing herein is to be construed as an admission that the embodiments described herein are not entitled to antedate such disclosure by virtue of prior invention.

DRAWINGS REFERENCE NUMERALS Prior Art

-   100 Prior Art Condenser Fan System -   106 Prior Art Condenser -   112 Prior Art Pressure Sensor -   114 Prior Art Expansion Valve -   116 Prior Art Evaporator -   118 Prior Art Compressor I -   119 Prior Art Compressor II -   120 Prior Art Condenser Fan -   122 Prior Art Controller -   124 Thermal Bulb -   140 Prior Art Variable Frequency Drive (VFD) -   *********************************** -   200 Air-cooled Expansion Valve based Condenser Fan System     Configuration I -   300 Air-cooled Expansion Valve based Condenser Fan System     Configuration II -   400 Air-cooled Expansion Valve based Condenser Fan System     Configuration III -   500 Condenser based Condenser Fan System Configuration I -   600 Condenser based Condenser Fan System Configuration II -   700 Compressor based Condenser Fan System Configuration I -   800 Compressor based Condenser Fan System Configuration II -   206 Condenser -   212 How Meter I -   214 Expansion Valve -   216 Evaporator -   218 Compressor I -   219 Compressor II -   220 Condenser Fan -   222 Controller -   224 Thermal Bulb -   312 Flow Meter II -   314 Pressure Sensor I -   324 Pressure Sensor II -   428 Temperature Sensor II -   430 Temperature Sensor III -   532 Temperature Sensor IV -   534 Pressure Sensor III -   632 Temperature Sensor V -   634 Temperature Sensor VI -   708 Compressor Status and Speed Device I -   738 Pressure Sensor IV -   808 Compressor Status and Speed Device II -   838 Temperature Sensor VI

DETAILED DESCRIPTION

Seven possible embodiments of the condenser fan control system are provided herein. All of the possible configurations shown in the described embodiments use the same existing refrigeration system. The existing components that are the same are thus labeled identically in all of the figures. Components that are not labeled identically in the figures are components that are different in each embodiment (such as the added temperature or pressure sensors). The existing refrigeration system can be any already installed refrigeration system. In the embodiments illustrated in the figures, air-cooled condensers with condenser fans are employed. It should be noted however that in other embodiments not illustrated herein, the heating, ventilating, and air-conditioning (HVAC) system may instead be equipped with water-cooled condenser(s) instead of air-cooled condensers. In such embodiments, control valves that open and close to control a water flow rate are used.

The existing refrigeration system shown in FIGS. 2-4 is comprised of compressor I 218 and II 219, condenser 206, flow meter 212, expansion valve 214, condenser fan 220, thermal bulb 224, and VFD II 240 connected in series in a typical refrigerant loop. The existing refrigeration system shown in FIGS. 5-6 is comprised of evaporator 216, compressor I 218 and II 219, condenser 206, expansion valve 214, condenser fan 220, thermal bulb 224, and VFD II 240 connected in series in a typical refrigerant loop. The existing refrigeration system shown in FIGS. 7-8 is comprised of condenser 206, expansion valve 214, evaporator 216, compressor I 218 and II 219, condenser fan 220, thermal bulb 224, and VFD II 240 connected in series in a typical refrigerant loop. In addition, FIGS. 7 & 8 also include compressor speed and status device I 708 (for FIG. 7), or II 808 (for FIG. 8) which can be implemented in or be an already existing component of the refrigerant loop. It should also be noted that the refrigeration system shown in all of the figures is representative of an existing refrigeration system and is not limited to the stated components and could include more or fewer components than those mentioned herein. For example, in embodiments not illustrated, the existing refrigeration system may have only one compressor, or more than two. In all of the embodiments and figures described, the compressor(s) employed can be of any type. Thus, for example, in some embodiments the compressors may be single stage constant speed compressors, while in other embodiments they may be multiple speed/multiple stage compressors or variable capacity/variable speed compressors. In all of the embodiments disclosed herein, as in the prior art, the thermal bulb is configured in connection with and functions to open and close the expansion valve to control the amount of refrigerant that flows into the evaporator.

The purpose of the refrigeration system is to remove heat from an area that is low temperature into an area that is high temperature and thus cool a space. As is typical in refrigerant systems such as the one shown in the figures, cold refrigerant flows through evaporator 216 where it cools a space by absorbing heat. This heat is then transferred to condenser 206 via compressors I 218 and II 219. Compressors I 218 and II 219 have a suction line that pulls the low pressure and low temperature refrigerant from evaporator 216, and includes a discharge line to compress the refrigerant into a high temperature and high pressure vapor which is then pushed into condenser 206. In condenser 206, the heat is removed and the vapor cools into a liquid. Condenser fan 220 is located in the middle of condenser 206 and blows outdoor air across the coils of condenser 206 to cool and condense the refrigerant.

It should be noted that while the refrigeration system shown in the Figs. herein has multiple compressors (218 and 219), in other embodiments not illustrated the refrigeration system may only have one compressor. Expansion valve 214 is installed in the liquid line between condenser 206 and evaporator 216 and functions to reduce the high pressure, high temperature refrigerant from the condenser line into a low pressure and low temperature refrigerant that flows into evaporator 216. The refrigerant then proceeds along the flow line through evaporator 216 where the liquid is heated and becomes a vapor. Variable frequency drive (VFD) 240 is configured to drive condenser fan 220. The VFD is an optional component, however. In some embodiments, VFD 240 is not included. In other embodiments, such as those shown in FIGS. 7 and 8 of the drawings illustrated herein, a compressor status and speed device is configured within the refrigerant loop that functions to send to controller 222 a signal relaying the speed of the compressor(s) or the compressor status (indicating the number of compressors in the refrigeration system that are currently in operation).

In the first three embodiments shown in FIGS. 2-4, flow meter 212 is also included as part of the existing refrigeration system (systems 200-400). Flow meter 212 may be a component of the already existing refrigeration system or can be installed in the existing refrigeration system along with controller 222 and the pressure and/or temperature sensors. Flow meter 212 is configured with controller 222 and measures the flow rate of the refrigerant flowing through the existing HVAC system. It should be noted that other components can be used in place of flow meter 212. For example, in some embodiments a VFD can be implemented instead of flow meter 212 to measure and send to the controller the refrigerant flow rates of the refrigeration system.

The first three embodiments of the system (shown in the Figs. by systems 200, 300, and 400 in FIGS. 2-4) are called the expansion valve based configurations because the expansion valve of the HVAC system is either directly or indirectly controlled at a maximum open position. Keeping the expansion valve at its maximum open position functions to both increase the efficiency and reduce the power of the compressor. System configurations 200, 300, and 400 are thus most suitable for use in heating, ventilation, and air-conditioning (HVAC) refrigeration systems such as that illustrated in the figures as well as in rooftop units, split units, or CRAC (computer room air conditioning) units.

In the first embodiment of the disclosure, herein referred to as expansion valve based configuration I 200 and shown in FIG. 2 of the drawings, the condenser fan is controlled to directly maintain the position of expansion valve 214 at a position close to the maximum. This position is measured or given by controller 222. In the method of the embodiment, controller 222 is configured within the existing refrigeration system in connection with expansion valve 214 and is configured to collect signal information on the valve position of the expansion valve. In the figure shown, the existing refrigeration system comprises flow meter 212, evaporator 216, condenser 206, condenser fan 220, compressors I 218 and II 219, expansion valve 214, thermal bulb 224, and variable frequency drive (VFD) II 240 (this VFD is used to control condenser fan 220).

Controller 222 is implemented so that it is configured in communication with flow meter 212, VFD II 240 and compressors I 218 and II 219 of the existing refrigeration system. Controller 222 is pre-programmed with a critical refrigerant flow rate parameter. It is also configured with an expansion valve setpoint position. The valve is at a nearly fully open position at this setpoint (generally 95% open, but this rate is adjustable). Controller 222 compares the measured flow rate (sensed by flow meter 212) with the pre-programmed critical flow rate parameter. This comparison determines the on/off status of condenser fan 220. If the refrigerant flow rate is less than the critical flow rate, controller 222 is configured to command condenser 220 off. If the refrigerant flow rate is greater than the critical flow rate, controller 222 is configured to activate condenser fan 220 and then modulate the speed of the fan so that the position of expansion valve 214 is maintained at the pre-programmed setpoint position.

In the second and third configurations shown in the embodiments illustrated in FIGS. 3 and 4 (systems 300 and 400), the position of expansion valve 214 is indirectly maintained at the maximum open valve position. In the method for the configuration shown in FIG. 3, controller controls the speed of condenser fan 220 to maintain a condensing pressure measurement at a plurality of setpoint values based on a comparison of the system refrigerant flow rate with a critical flow rate. In the method for the configuration shown in FIG. 4, the controller controls the speed of condenser fan 220 to maintain a condensing temperature measurement at a plurality of setpoint values based on a comparison of the refrigerant flow rate with the critical flow rate.

In the second embodiment, herein referred to as expansion valve based configuration II 300 and shown in FIG. 3 of the figures, controller 222 measures the pressure across expansion valve 214 of the refrigeration system. As in the first configuration (system 200), controller 222 is configured in communication with an existing refrigeration system comprised of compressors 218 and 219, condenser fan 220 (in condenser 206), evaporator 216, expansion valve 214, thermal bulb 224 and VFD II 240. Flow meter 312 is also configured in the HVAC system in connection with controller 222. In some embodiments the flow meter can be installed into the existing refrigeration system while in others it may be a part of the already existing refrigeration system.

Controller 222 is configured to receive a signal indicating the valve position of expansion valve 214 of the existing system. At least one pressure sensor is also implemented in the existing system at the inlet of the expansion valve to sense the pressure in the liquid line. In FIG. 2, pressures sensors 314 and 324 are provided at the inlet and outlet of expansion valve 214. They are configured to measure the pressure across expansion valve 214 and to send the measured pressure value (P_(cond)) to controller 222. Flow meter 312 is configured in the outlet pipeline of condenser 206, however it is not limited to this location. Flow meter 312 is configured to measure the flow rate of the refrigerant in the liquid pipeline and to send this information to controller 222.

Once implemented in the refrigeration system, controller 222 is programmed with measurements obtained from the system including a critical flow rate value, a saturated pressure at the evaporative temperature (P_(evaporator)) from evaporator 216, and a sum of the pressure loss from expansion valve 214 and the pressure loss from the liquid line of the HVAC system at the critical flow rate (ΔP). Using the measured flow rate obtained from flow meter 312 and the pre-programmed critical flow rate value, controller 222 is able to calculate for a refrigerant flow ratio (ω). This ratio (ω) is obtained by dividing the refrigerant flow rate over the design flow rate.

Based on the information, controller 222 can then calculate for the liquid condensing pressure setpoint of condenser 206, defined herein as (P_(set)), using the following equation:

P _(set) =P _(evaporator)+ω² ΔP

Wherein:

P_(set) represents the optimal condensing pressure set point (also called the optimal head pressure set point) of condenser 206. P_(evaporator) is the saturated pressure corresponding to the evaporative temperature (for most HVAC applications this temperature is 40° F., however this is adjustable depending on the type of system employed). ω is the relative flow rate of the refrigerant. It is a ratio of the refrigerant flow rate over the design flow rate. ΔP is the sum of the pressure loss of expansion valve 214 and the pressure loss in the liquid line under the design flow rate conditions.

Based on the calculated condensing pressure setpoint (P_(set)), controller 222 controls expansion valve 214 so that it is maintained at the maximum open position and thus ensures the lowest head pressure. Controller 222 then compares the refrigerant flow rate with the critical flow rate (the critical value is generally 20%, but this value depends on the system). If the refrigerant flow rate is less than the critical flow rate minus a control band, controller 222 keeps condenser fan 220 off.

If the refrigerant flow rate value is higher than the critical flow rate value, controller 222 is programmed to activate condenser fan 220. When condenser fan 220 is activated, controller 222 modulates the speed of fan 220 to maintain the condensing pressure (P_(cond)) at the condensing pressure setpoint (P_(set)). The particular method in which controller 222 maintains the setpoint depends on the configuration. In embodiments in which air-cooled condensers are employed, such as that shown in the illustrations, controller 222 activates or inactivates condenser fan 220 to maintain the setpoint. In embodiments in which water-cooled condensers are employed, controller 222 can open/close a cooling liquid control valve to maintain the calculated set point. In embodiments in which constant speed fans and/or two position control valves are employed, controller 222 can control the fans and/or the position of the valve so that the set point value is maintained. Finally, in embodiments in which at least one VFD is employed to control the speed of condenser fan 220, controller 222 can maintain the setpoint by controlling the speed of the condenser fan at the desired rate.

A third embodiment of the invention, herein referred to as expansion valve based configuration III 400 and shown in FIG. 4 of the figures, is similar to expansion valve based configuration II 300 shown in FIG. 3 of the figures. As in the embodiment illustrated in FIG. 3, controller 222 is configured into an existing refrigeration system comprised of compressors I 218 and II 219, condenser fan 220 (in condenser 206), evaporator 216, expansion valve 214, flow meter 212, thermal bulb 224, and VFD 240. The difference between the embodiment illustrated in FIG. 3 and that shown in FIG. 4 is that in the third embodiment (configuration III 400), temperature sensor II 428 and III 430 are configured in communication with controller 222 rather than pressure sensor I 314 and II 324. While there are two temperature sensors illustrated in the Figs., in other embodiments only one temperature sensor is required (in order to collect a condensing temperature value (T_(cond))). In FIG. 4, the two temperature sensors are installed within the refrigeration system at the inlet and outlet of expansion valve 214. Temperature sensors 428 and 430 measure the temperature at the inlet and outlet of expansion valve 214 and send the collected temperature information to controller 222.

Like in the art prior configuration, once it is implemented in the refrigeration system, controller 222 is programmed with measurements obtained from the system including a critical flow rate value, a saturated pressure corresponding to the evaporative temperature (P_(evaporator)) from evaporator 216, and a sum of a pressure loss from expansion valve 214 and a pressure loss from the liquid line at the critical flow rate (ΔP). Using the measured flow rate obtained from flow meter 212, and the pre-programmed critical flow rate value, controller 222 calculates for a refrigerant flow ratio (ω) by dividing the refrigerant flow rate over the design flow rate.

Based on the information, controller 222 can then calculate for the condensing liquid pressure setpoint of condenser 206, defined herein as P (set), using the following equation:

P _(set) =P _(evaporator)+ω² ΔP

Wherein:

P_(set) represents the condensing liquid pressure set point of condenser 206. P_(evaporator) is the saturated pressure corresponding to the evaporative temperature (for most HVAC applications this temperature is 40° F., however this is adjustable depending on the type of system employed). ω is representative of the relative flow rate of the refrigerant. It is a ratio of the refrigerant flow rate over the design flow rate. ΔP is the sum of the pressure loss of expansion valve 214 and the pressure loss in the liquid line under the design flow rate conditions. In configuration III 400, an additional step can be taken to find the condensing liquid temperature setpoint (T_(set)). The condensing liquid temperature setpoint (T_(set)) is the saturated temperature of the refrigerant at the condensing liquid pressure set point (P_(set)), the condensing subcooled liquid temperature setpoint can be found by subtracting the refrigerant sub-cooled temperature from the condensing liquid temperature set point (T_(set)). In applications like configuration III 400, the subcooled temperature correction is not necessary if the temperature sensor measures the refrigerant temperature inside the condenser.

Based on the calculated liquid condensing temperature setpoint (T_(set)), controller 222 indirectly controls expansion valve 214 so that it is maintained at the maximum open position and can thus ensure that the lowest head pressure is obtained. Controller 222 then compares the refrigerant flow rate with the critical flow rate (the critical value is generally 20%, but this value depends on the system in which controller 222 is employed). If the refrigerant flow rate is less than the critical flow rate minus a control band, controller 222 keeps condenser fan 220 off. If the refrigerant flow rate is higher than the critical flow rate, controller 222 is programmed to activate condenser fan 220. When condenser fan 220 is activated, controller 222 modulates the speed of the fan to maintain the liquid condensing temperature (T_(cond)) at the liquid condensing temperature setpoint (T_(set)). The particular method in which controller 222 maintains the liquid condensing temperature setpoint (T_(set)) depends on the configuration. In embodiments in which air-cooled condensers are employed, such as that shown in the illustrations, controller 222 activates or inactivates condenser fan 220 to maintain the setpoint value. In embodiments in which water-cooled condensers are employed, controller 222 can open/close the cooling liquid control valve to maintain the calculated set point value. In embodiments in which constant speed fans and/or two position control valves are employed, controller 222 can control the fans and/or the position of the valve so that the set point value is maintained. In embodiments in which at least one variable speed drive is employed to control the speed of condenser fan 220, controller 222 can maintain the setpoint by controlling the speed of condenser fan 220 at the desired rate.

The systems shown in FIGS. 5 and 6 (systems 500 and 600) are referred to herein as the Condenser Based Condenser Fan System Configurations. The following summarizes these embodiments. In a fourth embodiment of the invention (shown in FIG. 5 and system 500), controller 222 is configured to determine the pressure setpoint of condenser 206 based on measurements of the condensing pressure and ambient air temperature. These measurements are used to find the load ratio (β) and condensing pressure setpoint. The pressure setpoint is maintained by modulating the fan speed and/or the condenser fan on or off based on a comparison of the measured condensing pressure value and the condensing pressure setpoint. Condenser fan 220 is programmed to inactivate if the pressure of condenser 206 is less than the corresponding saturated pressure at the ambient temperature plus 5° F. (this amount is adjustable so can be at or around this value). In a fifth embodiment of the invention (as shown in FIG. 6 by system 600), controller 222 is configured to determine the temperature setpoint of the condenser based on variables pre-programmed into controller 222 as well as the condenser and ambient temperature measurements. These measurements are used to find the load ratio and condensing liquid temperature setpoint. The temperature setpoint is maintained by modulating the condenser fan on/off, and/or by modulating the fan speed based on a comparison of the measured condensing temperature value and the condensing temperature setpoint. In this embodiment, the condensing temperature setpoint is the temperature value that corresponds to the pressure at the condensing pressure setpoint. Condenser fan 220 is programmed to inactivate if the pressure of condenser 206 is less than the corresponding saturated pressure at the ambient temperature plus 5° F. (this amount is adjustable). The configurations shown in the fourth (FIG. 5) and fifth (FIG. 6) embodiments (systems 500 and 600) are thus particularly suitable for implementation in split HVAC units, computer room air conditioning (CRAC) units, and industrial and commercial refrigeration systems.

The following describes the fourth embodiment in more detail. In the fourth embodiment, illustrated in FIG. 5, Controller 222, ambient air temperature sensor IV 532, and pressure sensor IV 534 are implemented in the existing refrigeration system. The existing refrigeration system in this embodiment comprises condenser 206, expansion valve 214, evaporator 216, compressor I 218 and II 219, condenser fan 220, thermal bulb 224, and VFD II 240. Also, as in the other embodiments, this refrigeration system is an example of a refrigeration system in which the invention could be implemented and so is not limited to the parts described herein and may thus include fewer or additional components.

Once implemented in the refrigeration system, controller 222 is programmed with the system variables obtained from the refrigeration system. These variables are a condenser fan speed ratio (ω), a ratio of a speed of said condenser fan over a design speed of said condenser fan; a design condenser split temperature value from said refrigeration system (ΔT), an on/off heat transfer coefficient ratio of said condenser fan (li, a suction pressure value (P_(suc)) from the evaporator, and a sum of the pressure loss value across the liquid pipeline, suction pipeline, and the expansion valve of the refrigeration system under the design flow rate (ΔP_(d)).

Pressure sensor III 534 is configured in the liquid line of the existing refrigeration system in the pipe at the outlet of condenser 206 and measures the pressure of the refrigerant in the liquid line after it leaves the condenser. Pressure sensor III 534 is configured in communication with and sends the collected pressure measurements to controller 222. Temperature sensor IV 532 measures the temperature of the ambient air and is configured in communication with and operable to send to controller 222 the collected ambient air temperature measurements.

The following details the method for finding the pressure setpoint of the system in condenser based fan system configuration I 500 (FIG. 5 of the drawings). In the method, the system cooling load ratio (β) of condenser 206 is first calculated by controller 222 based on the collected temperature and pressure measurements. Then, the load ratio is employed in an algorithm to determine the liquid pressure setpoint of the refrigerant in condensor 206 (P_(cond,set)). The measured variables are the speed of condenser fan 220, the ambient air temperature (obtained from temperature sensor IV 532), and the condensing pressure (P_(cond)) as measured by pressure sensor III 534.

In the method for the embodiment, controller 222 receives ambient temperature measurements from temperature sensor IV 532 and measurements of the refrigerant pressure from pressure sensor III 534. In some embodiments (not illustrated in the figures), controller 222 may also obtain the temperature of the refrigerant inside condenser 206 from a temperature sensor located in the condenser. Using the following variables such as the preprogrammed fan speed ratio (ω), and heat transfer coefficient ratio (α), controller 222 calculates for the condenser load ratio (β):

$\beta = \left\{ \begin{matrix} {\frac{T_{cond} - T_{amb}}{\Delta \; T}\omega^{0.76}} & {\omega > 0.1} \\ {\frac{T_{cond} - T_{amb}}{\Delta \; T}\alpha} & {\omega \leq 0.1} \end{matrix} \right.$

Wherein:

β—condenser load ratio ΔT—design condenser split temperature (from the existing rooftop unit) ω—condenser fan speed ratio (ratio of the fan speed over the design speed) α—the fan on/off heat transfer coefficient ratio T_(cond)—condensing temperature. (This variable is measured directly by temperature sensor IV 634 in condenser based configuration II 600 (FIG. 6). It can be determined by finding the saturated refrigerant temperature at the measured head pressure for condenser based configuration I 500 (FIG. 5)).

In the embodiment shown in configuration 1 500, once the condenser load ratio (β) is found, it can then be used to find the condensing pressure setpoint of the refrigerant in the condenser (P_(cond,set)). Controller 222 uses the following algorithm to calculate for the condensing pressure set point:

P _(cond,set) =P _(suc)+β² ΔP _(d)

Wherein:

P_(cond,set)—The liquid pressure set point of the refrigerant in the condenser P_(suc)—The suction pressure. This is defined as the saturation pressure that corresponds to the evaporating temperature. ΔP_(d)—The sum of the pressure loss across the expansion valve, liquid pipe line, and suction pipe line under the design refrigerant flow rate. β—The condenser load ratio

If the condensing pressure measurement is less than the corresponding saturated pressure at the ambient temperature plus 5° F. (this temperature value is adjustable), controller 222 is programmed to disable the operation of condenser fan 220. If the condensing pressure measurement (P_(cond)) is higher than the corresponding saturated pressure value at said ambient temperature value plus 5° F. (this temperature value is adjustable and not limited to 5° F.), controller 222 is programmed to activate the condenser fan. Controller 222 is configured to modulate the speed of condenser fan 220 to maintain the condensing pressure measurement (P_(cond)) at the condensing pressure setpoint (P_(cond,set)) when the condensing pressure measurement is greater than the corresponding saturated pressure value at said ambient temperature value plus 5° F. (adjustable).

In the embodiment shown in FIG. 6 and described by system 600, ambient air temperature measurements, condensing temperature measurements, as well as the variables already pre-programmed in Controller 222 (such as the fan speed ratio and heat transfer coefficient ratio), are used to find a condensing pressure setpoint (P_(cond,set)) and a condensing temperature setpoint (T_(cond,set)) which can be used to maintain a condensing temperature value at a plurality of condensing temperature set points. Controller 222 can solve for the load ratio (β) as shown above in the description for system 500). This load ratio (β) is used to determine the condensing pressure setpoint (P_(cond,set)) using the same method and equation shown in the description for the fourth embodiment (system 400). Once the condenser load ratio (β) is calculated, controller 222 is programmed to calculate for the condensing pressure setpoint (P_(cond,set)) as described in the embodiment for system 500. In the embodiment for system 600, however, the condensing temperature set point must be found once the condensing pressure setpoint is known. Controller 222 is programmed to find the temperature setpoint of the liquid refrigerant (T_(cond,set)) as the saturated temperature under the condensing pressure setpoint (P_(cond,set)).

If the condensing temperature measurement (T_(cond)) is less than the ambient temperature plus 5° F. (this temperature value is adjustable), controller 222 is programmed to disable the operation of condenser fan 220. If the condensing temperature measurement (T_(cond)) is higher than the ambient temperature value plus 5° F. (this temperature value is adjustable), controller 222 is programmed to activate the condenser fan. Controller 222 is configured to modulate the speed of condenser fan 220 to maintain the condensing temperature measurement (T_(cond)) at the condensing temperature setpoint (T_(cond set)) when the condensing temperature measurement is higher than the ambient temperature value plus 5° F. (this temperature value is adjustable).

Thus, in both systems 500 and 600 (FIGS. 5 and 6), controller 222 is configured to modulate condenser fan 220 and/or (in the case of a water-cooled condenser) modulate a control valve to maintain the set points. In the fourth embodiment (system 500), this setpoint is the condensing pressure setpoint. In the fifth embodiment (system 600), this setpoint is the condensing temperature setpoint.

In a sixth and seventh embodiment of the disclosure, shown in FIGS. 7 and 8 of the drawings, controller 222 is also configured to control the condenser fan of the HVAC system to maintain the condensing temperature or condensing pressure at a plurality of condensing liquid temperature setpoint (T_(set)) or condensing liquid pressure setpoints (P_(set)). The difference is that in this configuration the speed of compressors I 118 and II 119 is used to determine the set points used to control the condenser fan (in systems with air-cooled condensers) or the position of the condenser control valve (in systems equipped with water-cooled condensers not illustrated in the Figs.). As such, the sixth and seventh embodiments are referred to herein as the Compressor Based Condenser Fan System Configurations. The method for these embodiments is similar to the method in the second and third embodiments. The main difference is that in these embodiments the speed of the compressors or the compressor status (the number of compressors in operation) is used to determine the refrigerant flow rate instead of a flow meter. In the Figs., the compressor speed/status is measured by a compressor speed and status device. Data collected from the device is used to determine the liquid condensing pressure setpoint (P_(set)).

In the embodiment illustrated in FIGS. 7 and 8 (systems 700 and 800), controller 222 is implemented in the existing refrigeration system comprising condenser 206, expansion valve 214, evaporator 216, compressors I 218 and II 219, condenser fan 220, thermal bulb 224, VFD II 240. If not already a part of the existing refrigeration system, compressor status & speed device I 708 (when using the configuration illustrated in FIG. 7) or II 808 (when using the configuration illustrated in FIG. 8) is installed to measure the speed of compressors I 218 and II 219. There are no differences between compressor status & speed device I 708 and II 808. The difference in the numbering of the Figs. is merely for illustrative purposes to show that a compressor status and speed device is intended to be included in the embodiments shown in FIGS. 7 and 8 but not in the other described embodiments. Compressor status & speed device I 708 and II 808 are configured to receive a signal of the compressor speed or status and send to controller 222 a compressor speed ratio (ω) that controller 222 uses to calculate for the liquid condensing pressure set point (P_(set)). The compressor speed ratio (ω) for compressor based condenser valve configurations I and II 700 and 800 is defined as the ratio of the speed of the compressor over a design speed of the compressor. As an alternative to the compressor speed ratio (ω), in other embodiments of compressor based condenser valve configurations I and II 700 and 800, the controller can instead be configured to find a compressor status ratio where (ω) represents the ratio of the number of compressors in the HVAC system that are in active operation over the total number of compressors (in the HVAC system).

Controller 222 is also programmed with a plurality of variables for the HVAC system in which it is installed. These variables comprise a design refrigerant flow rate, the compressor speed ratio (ω) from compressor status & speed devices I 708 and II 808 when using the configuration illustrated in FIG. 8), a measurement of the saturated pressure corresponding to the evaporative temperature (P_(evaporator)) from evaporator 216, and a sum of a pressure loss from expansion valve 214 and the liquid and suction lines of the HVAC system under the design flow rate conditions (ΔP). Controller 222 then calculates for a condensing pressure set point (P_(set)) based on the selected ratio (ω), (either the compressor speed or compressor status), a measurement of the saturated pressure corresponding to the evaporative temperature (P_(evaporator)) from the evaporator of the HVAC system, and a sum of a pressure loss from the expansion valve and liquid and suction lines of the HVAC system under the design flow rate conditions (ΔP). Controller 222 calculates for the condensing pressure set point (P_(set)) using the following equation:

P _(set) =P _(evaporator)+ω² ΔP

Wherein:

P_(set) represents the liquid pressure set point, or the optimal pressure set point of condenser 206. P_(evaporator) is the saturated pressure corresponding to the evaporative temperature (for most HVAC applications this temperature is 40° F., however this is adjustable depending on the type of system employed). ω is representative of the compressor speed ratio. In some embodiments (like those shown in FIGS. 7 and 8, it can be defined as the relative speed of the compressor/the design speed of the compressor. In other embodiments it is defined as the compressor operating status ratio (the number of compressors in operation over the total number of compressors in the HVAC system). In some embodiments, like those shown in FIGS. 2 & 3, it is representative of the refrigerant flow rate over the design flow rate. ΔP is the sum of the pressure loss of expansion valve 214 and the liquid line under the design flow rate conditions.

The controller is configured to activate condenser fan 220 when the flow rate is higher than the critical flow rate. In the same way, condenser fan 220 is deactivated when the refrigerant flow rate is lower than the critical refrigerant flow rate minus a control band. The critical flow rate value is generally 20%, but this value depends on the system in which controller 222 is employed).

Compressor based configurations I & II 700 & 800 are very similar but use different sensors. This difference results in differences in the method of controlling condenser fan 220 to maintain the liquid condensing pressure setpoint (P_(set)). In condenser based compressor fan speed configuration I (system 700), pressure sensor VI 738 is configured at the outlet and measures the pressure of the refrigerant exiting condenser 206. The pressure sensor then sends the collected pressure measurement (P_(cond)) to controller 222. When the refrigerant flow rate in the HVAC system is higher than the critical flow rate, Controller 222 is configured to modulate the speed of the condenser fan so that the condensing pressure measurement (P_(cond)) is maintained at the condensing liquid pressure setpoint (P_(set)), which can be a plurality of values.

In compressor based fan speed configuration II (system 800 as shown in FIG. 8), temperature sensor VI 838 is configured in the outlet pipes of condenser 206 of the existing refrigeration system. The temperature sensor measures the temperature of the refrigerant exiting condenser 206 and sends the collected temperature measurement (T_(cond)) to controller 222. Controller 222 then uses the calculated liquid condensing pressure setpoint (P_(set)), to calculate for a liquid condensing temperature setpoint (T_(set)). The (T_(set)) value is found as the saturated temperature of the refrigerant under the condensing pressure setpoint (P_(set)). If the temperature sensor does not directly measure the saturated temperature inside of the condenser, the condensing temperature setpoint needs to be corrected. This correction is achieved by subtracting the condensing temperature setpoint minus a subcooling temperature. When the refrigerant flow rate is higher than the critical refrigerant flow rate, controller 222 is configured to modulate the speed of condenser fan 220 so that the condensing temperature measurement (T_(cond)) is maintained at the condensing liquid temperature setpoint (T_(set)), which can be a plurality of values.

In summary, the particular method in which controller 222 maintains the liquid pressure setpoint (P_(set)) (in configurations like the one shown in FIG. 7) or the liquid condensing temperature setpoint (T_(set)) (in configurations like the one shown in FIG. 8) depends on the configuration of the HVAC (heating, ventilating, and air-conditioning) system in which controller 222 is implemented. In embodiments in which air-cooled condensers are employed, such as that shown in the illustrations, controller 222 activates or inactivates condenser fan 220 to maintain the setpoint value. In embodiments in which water-cooled condensers are employed, controller 222 can open/close the cooling liquid control valve to maintain the calculated set point value. In embodiments in which constant speed fans and/or two position control valves are employed, controller 222 can control the fans and/or the position of the valve so that the set point values are maintained. In embodiments in which at least one variable speed drive is employed to control the speed of condenser fan 220 (as shown in FIGS. 7 and 8), controller 222 can maintain the setpoint by controlling the speed of condenser fan 220.

The above-described features and advantages of the present disclosure thus improve upon aspects of those systems and methods in the prior art. 

What is claimed is:
 1. A method of controlling a condenser fan to maintain a valve position of an expansion valve of a heating, ventilating, and air-conditioning (HVAC) system at a valve position setpoint, said method comprising: providing a controller in communication with said expansion valve of said HVAC system, said controller operable to receive a signal indicating said valve position; providing a refrigerant flow rate sensing device in communication with said HVAC system and said controller and operable to measure a refrigerant flow rate of said HVAC system; configuring said controller with a critical flow rate value and said valve position setpoint; comparing, by said controller, said refrigerant flow rate with said critical flow rate value; modulating, by said controller, said condenser fan to maintain said valve position of said expansion valve at said valve position setpoint when said refrigerant flow rate is higher than said critical flow rate value.
 2. The method of claim 1, further comprising inactivating, by said controller, said condenser fan when said refrigerant flow rate value is lower than said critical flow rate value.
 3. The method of claim 1, further comprising activating, by said controller, said condenser fan when said refrigerant flow rate value is higher than said critical flow rate value.
 4. A method of controlling a condenser fan of a heating, ventilating, and air-conditioning (HVAC) system to maintain a liquid condensing measurement of a refrigerant at a plurality of condensing measurement setpoints, said HVAC system comprising at least one evaporator, expansion valve, condenser, and compressor configured in a refrigerant circuit, said method comprising: providing a condensing measurement device in communication with said condenser and controller and configured to provide a condensing measurement; providing a sensing device in communication with said controller and operable to sense a flow rate value of said refrigerant; determining, by said controller, a system load ratio (ω) for said HVAC system; programming said controller with a plurality of variables for said HVAC system comprising a critical flow rate value of said refrigerant, a design flow rate value of said refrigerant, said system load ratio value (ω), a subcooling liquid temperature value for said refrigerant, a saturated pressure measurement (P_(evaporator)) from said evaporator; and a sum of a pressure loss value from said expansion valve and a pressure loss value in a liquid line of said refrigerant circuit at said design flow rate value (ΔP); determining, by said controller, a condensing pressure setpoint (P_(set)) of said condenser based on said plurality of variables, wherein P_(set)=P_(evaporator)+ω²ΔP; modulating, by said controller, said speed of said condenser fan to maintain said condensing measurement value at said plurality of condensing measurement setpoints when said flow rate value of said refrigerant is higher than said critical flow rate value.
 5. The method of claim 4, wherein said condensing measurement device is a pressure measurement device, said condensing measurement is a pressure measurement at said liquid line of said refrigerant circuit, and said plurality of condensing measurement setpoints is said condensing pressure setpoint (P_(set)).
 6. The method of claim 4, wherein said condensing measurement device is a temperature measurement device, said condensing measurement is a temperature measurement at said liquid line of said refrigerant circuit, and said plurality of condensing measurement setpoints is a saturated temperature value of said refrigerant at said condensing pressure setpoint (P_(set)).
 7. The method of claim 4, wherein said sensing device is a flow meter and wherein determining, by said controller, said system load ratio (ω) for said HVAC system further comprises dividing, by said controller, said flow rate value over said design flow rate value.
 8. The method of claim 4, wherein said sensing device is a compressor status device operable to collect and transmit to said controller a signal indicating when said compressor is in an active state of operation and wherein determining, by said controller, said system load ratio (ω) for said HVAC system further comprises: providing at least one additional compressor in communication with said compressor status device and said controller; transmitting, by said compressor status device, said compressor status signal for said at least one additional compressor to said controller; and dividing, by said controller, a sum of said compressor status signal for said compressor and said compressor status signal for said at least one additional compressor over a sum of said compressor and said at least one additional compressor.
 9. The method of claim 4, wherein said sensing device is a compressor speed device operable to collect and transmit to said controller a signal indicating a speed of said compressor and wherein determining, by said controller, said system load ratio (ω) for said HVAC system further comprises: providing said controller with a design speed for said compressor; transmitting, by said compressor status device, said signal indicating said speed of said compressor to said controller; and dividing, by said controller, said speed of said compressor over said design speed.
 10. The method of claim 4, further comprising inactivating, by said controller, said condenser fan when said refrigerant flow rate value is lower than said critical flow rate value.
 11. The method of claim 4, further comprising activating, by said controller, said condenser fan when said refrigerant flow rate value is higher than said critical flow rate value.
 12. A method of controlling a condenser fan of a condenser of a heating, ventilating, and air-conditioning (HVAC) system to maintain a liquid condensing measurement of a refrigerant at a plurality of condensing measurement setpoints, said HVAC system also comprising an evaporator and expansion valve connected in a refrigerant circuit, said method comprising: providing a controller in communication with said HVAC system; providing an ambient air temperature sensor in communication with and operable to measure and send to said controller an ambient air temperature value (T_(amb)); providing a condensing measurement device in communication with said controller and operable to measure and send to said controller a condensing measurement value; determining, by said controller, a condensing temperature value (T_(cond)); programming said controller with a ratio of a speed of said condenser fan over a design speed of said condenser fan (ω), a design condenser split temperature value of said HVAC unit (ΔT), an on/off heat transfer coefficient ratio for said condenser fan (α), a suction pressure value (P_(suc)) of said evaporator, a subcooling liquid temperature value of said refrigerant, and a sum of a pressure loss value across a liquid line and suction line of said refrigerant circuit and said expansion valve of said HVAC unit under a design refrigerant flow rate (ΔP_(d)); determining, by said controller, a cooling load ratio (β) of said condenser, wherein: $\beta = \left\{ \begin{matrix} {\frac{T_{cond} - T_{amb}}{\Delta \; T}\omega^{0.76}} & {\omega > 0.1} \\ {\frac{T_{cond} - T_{amb}}{\Delta \; T}\alpha} & {\omega \leq 0.1} \end{matrix} \right.$ determining, by said controller, a condensing liquid pressure set point (P_(cond.set)) of said refrigerant based on said cooling load ratio (β), wherein: P _(cond.set) =P _(suc)+β² ΔP _(d) determining, by said controller, a plurality of condensing measurement setpoints based on said condensing measurement value and said condensing liquid pressure set point (P_(cond.set)); modulating, by said controller, said speed of said condenser fan to maintain said condensing measurement at said plurality of condensing measurement setpoints when said condensing temperature value is higher than said ambient temperature value plus at or around 5° F.
 13. The method of claim 12, wherein determining, by said controller, said condensing temperature value (T_(cond)) further comprises: providing a temperature sensor in communication with and operable to measure and send to said controller said condensing temperature value (T_(cond)) of said refrigerant.
 14. The method of claim 12, wherein determining, by said controller, said condensing temperature value (T_(cond)) further comprises the steps of: providing a pressure sensor in communication with said condenser and controller operable to measure and send to said controller a condensing pressure value of said refrigerant; and determining a saturated temperature of said refrigerant at said condensing pressure value.
 15. The method of claim 12, further comprising: providing a pressure sensor in communication with and operable to measure and send to said controller a liquid pressure measurement value (P_(cond)); determining, by said controller, said plurality of condensing measurement setpoints as a plurality of condensing pressure setpoints (P_(set)); modulating, by said controller, said speed of said condenser fan to maintain said pressure measurement value (P_(cond)) at said plurality of condensing measurement setpoints when said pressure measurement value (P_(cond)) is higher than said saturated pressure under said ambient temperature value plus at or around 5° F.
 16. The method of claim 12, further comprising: providing a temperature sensor in communication with and operable to measure and send to said controller said liquid temperature condensing measurement value (T_(cond)); determining, by said controller, said plurality of condensing measurement setpoints as a corresponding plurality of saturated refrigerant temperatures at said condensing liquid pressure setpoint (P_(cond.set)); modulating, by said controller, said speed of said condenser fan to maintain said temperature measurement value (T_(cond)) at said plurality of condensing temperature measurement setpoints when said temperature measurement value (T_(cond)) is higher than said ambient air temperature value plus at or around 5° F.
 17. The method of claim 15, further comprising inactivating, by said controller, said condenser fan when said condensing pressure value (P_(cond)) is less than said corresponding saturated pressure value at said ambient air temperature value plus at or around 5° F.
 18. The method of claim 15, further comprising activating, by said controller, said condenser fan when said condensing pressure value (P_(cond)) is higher than said corresponding saturated pressure value at said ambient temperature value plus at or around 5° F.
 19. The method of claim 16, further comprising activating, by said controller, said condenser fan when said condensing temperature value (T_(cond)) is higher than said ambient air temperature value plus at or around 5° F.
 20. The method of claim 16, further comprising inactivating, by said controller, said condenser fan when said condensing temperature value (T_(cond)) is less than said ambient air temperature value plus at or around 5° F. 